Include examples as notebooks in the documentation

[RemiLehe]

This PR uses nbsphinx so that we can use notebooks as source files in the Sphinx documentation.

The notebooks are then nicely formatted in the chosen style of the documentation. In addition, they are run by readthedocs as part of building the documentation - which provides one form of automatic testing. (The readthedocs build will fail if the notebook is obsolete and does not work anymore.)

In particular, our first example (gaussian laser) has now been converted in notebook format.

Future PRs will add more notebooks in the tutorials folder.

[AngelFP]

Looks good, thanks for this!